Pipeline pigs utilizing staples in a cloth backing have been used heretofore. The cloth with staples is a material available from the textile industry, and it is often referred to as carding cloth. It is formed of multiple layers of a backing cloth such as heavy ducking or the like. A number of U-shaped staples are driven through the cloth backing to position a set of points which points are so close together as to define an abrasive surface on the exterior of the pig. It has been discovered that the staple legs and points on the staple legs tend to chatter as the pig passes through a pipeline.
The points themselves bear against the wall of the pipe and thereby clean the pipe which is the intended purpose of the pig. However, the certain problems have arisen in the use of pipeline cleaning pigs of this sort. It appears now in retrospect that the cantilevered staple legs have tips which abrade the surface of the pipe but in so doing, they tend to chatter and set up oscilliations or even shock waves in the staples which cause rapid metal fatigue or wear. The metal fatigue might occur at any point in the U-shaped staple while the accelerated wear occurs from the tip or point of the staple. Accordingly, the life of the staples has been limited and of course when they collectively fatigue or fail, the pig must be taken out of service.
The present disclosure overcomes this problem. Heretofore, pigs have been constructed where the cloth backing of the carding cloth has been bonded into the elastomeric body of the pig. The staples have been left free so that the cantilevered tips have been unsupported. The present invention overcomes this infirmity. The present invention contemplates forming a pipeline pig wherein an external coating is placed on the pig which coating engulfs the cantilevered tips or points of the staples. This provides surrounding support and positional stability to the metal staples. This holds them in a relatively fixed position and thereby cuts down on chatter as they abrade against the surface of the pipe. In addition, the exterior coating surface is able to support some of the weight of the pig thereby reducing the point loading on the staples and thereby increasing the life of the pig. It is not necessary to make the pig heavy to achieve heavy loading on the tips of the staples because they are so small in cross sectional area at the point of contact that the loading is quite high, even with a very lightweight pig body. So to speak, it is not necessary that the points abrade so deeply into the film or accumulation of coatings in the pipes that the pipe is clean to bare metal; rather, a cleaning away of the accumulation of materials deposited by the flow in the pipe itself as exemplified by water scale is adequate.